The present disclosure relates to a technology for automatically controlling the light quantity of a light beam emitted from a light emitting element which acts as a light source of an optical scanning device (APC: Automatic Power Control).
An optical scanning device is a device for drawing a scanning line by emitting a light beam and installed in an image forming apparatus such as a printer, a projector or the like. A semiconductor laser is used as a light emitting element which acts as a light source of the optical scanning device. Even at the same drive current, the light quantity of a light beam emitted from the semiconductor laser differs depending on temperature. Thus, the APC is performed to bring the light quantity of the light beam emitted from the semiconductor laser to a target value.
An APC in an image forming apparatus is briefly described. A semiconductor laser which acts as a light source of an optical scanning device simultaneously emits light beams forward and backward. The light beam emitted forward is used to draw a main scanning line. Specifically, the light beam emitted forward is reflected by a polygon mirror disposed in front of the semiconductor laser and the main scanning line is drawn on a photoconductive drum by the reflected light beam.
The light beam emitted backward is used for the APC. Specifically, the light beam emitted backward is received by a photodiode disposed behind the semiconductor laser. During an APC period, the light quantity of the light beam emitted backward is feedback-controlled to a target value based on a signal output from the photodiode.
Since the light beam emitted forward and the one emitted backward are emitted from the same semiconductor laser, the light quantity is same. Accordingly, the light quantity of the light beam emitted forward can be feedback-controlled to the target value by feedback-controlling the light quantity of the light beam emitted backward based on the light quantity of the light beam emitted backward.
When the light beam emitted forward is reflected by the polygon mirror and received by the photodiode arranged behind the semiconductor laser during the APC period, the photodiode receives the light beam emitted forward and reflected by the polygon mirror in addition to the light beam emitted backward. This may lead to erroneous recognition that the light quantity of the light beam emitted backward reaches the target value although the light quantity of the light beam emitted backward has not reached the target value yet. The drawing of the main scanning line by the light beam, the light quantity of which has not reached the target value, causes a reduction in image quality.
Accordingly, an optical scanning device has been proposed which can perform an APC without being affected by the light beam reflected by the polygon mirror. This optical scanning device performs the APC based on a synchronization detection signal obtained by a light receiving element while avoiding a timing at which an incident angle of the light beam with respect to the polygon mirror becomes substantially 90°.
An object of the present disclosure is to provide an optical scanning device and an image forming apparatus capable of performing an APC without being affected by a light beam reflected by a mirror unit.